A multiphase alternating current system may be set up in either of two ways. In a conventional three-phase, three wire system, three power lines are used to connect a three-phase alternating current AC power source to a load. A first one of the power lines carries a current I.sub.1 and a voltage V.sub.13, where the voltage of the first power line is measured relative to the potential of a third power line, and a second power line carries a current I.sub.2 and a voltage V.sub.23, where V.sub.23 is measured relative to the potential of the third power line. The total power applied to the load, p, is known to be: EQU P=V.sub.13 I.sub.1 +V.sub.23 I.sub.2
This system is further characterized in that the voltage applied to each one of the first and second power lines is substantially in phase with the current being carried by that power line.
A similar multiphase system which has four lines is also known. In a three-phase, 4 wire system, a three-phase AC power source is connected to a load by means of a set of three power lines and a neutral, or reference, line. Each of the power lines carries a current and a voltage, where the voltage in each line is measured relative to the potential of the neutral line. The total power applied to the load by the multiphase system, P, is derived according to the equation: EQU P=V.sub.1 I.sub.1 +V.sub.2 I.sub.2 +V.sub.3 I.sub.3,
where V.sub.1 the voltage applied to a first power line; V.sub.2 is the voltage applied to a second power line; V.sub.3 is the voltage applied to a third power line; and I.sub.1, I.sub.2 and I.sub.3 represent the currents flowing through the first power line, the second power line, and the third power line, respectively. Once again, the voltage applied to any one of the power lines is substantially in phase with the current flowing through that power line.
In the three-phase, three-line system, the voltage applied to the first power line is 120.degree. out of phase with the voltage applied to the second power line. Similarly, in the three-phase, four-line system, the voltage applied to the first power line is 120.degree. C. out of phase with the voltage applied to the second power line and 240.degree. C. out of phase with the voltage applied to the third power line.
It is important to be able to measure the power demand on a multiphase AC power system like those described above. This may be done by connecting a watt/watthour meter to the multiphase system. A schematic illustration showing how to connect said watt/watthour meter to the multiphase system is presented in FIG. 1. The multiphase system is a three phase, four wire system connecting a three-phase AC generator 1 to a load 2 as previously described. Each of the three power lines 4, 5 and 6 is individually connected to the watt/watthour meter by one of the voltage input lines 7, where each of the lines 7 leads to one of a set of three voltage input terminals 8, 9 and 10. Each of the power lines is further connected to one of the current sensing coils 11, 12 and 13, where the current sensing coil is (a) inductively coupled to the power line so as to cause a current which is proportional to the current in the power line to flow through the current sensing coil and (b) electrically connected to the watt/watthour meter by one of a set of current input terminals 14, 15 and 16. Additionally, the neutral line 3 is connected to a reference voltage input terminal 3a by input line 7a. Each of the voltage input terminals is designated by the watt/watthour meter as being connected to a power line having a voltage of a specific phase applied thereto, and each of the current input terminals is designated as being connected to the same power line as a specific voltage input terminal.
The watt/watthour meter measures the voltage applied to a voltage input line which is connected to a designated voltage input terminal. Next, the watt/watthour meter measures the current flowing through the current sensing coil which is connected to the current input terminal which is designated as being connected to the same power line as the designated voltage input terminal. The measured current and the measured voltage are then multiplied to obtain a measure of power. The power in each phase of the system is individually measured, and all measured single-phase powers may then be added together to obtain the total power in the system.
It is essential that the voltage input line and the current sensing coil connected to a specific power line carrying a voltage and current of a given phase be connected to the voltage input terminal and current input terminal which are designated as receiving inputs from that specific power line. Otherwise, the watt/watthour meter will calculate an incorrect value for the power in that power line. For example, if the current sensing coil is connected to the wrong current input terminal, the watt/watthour meter will multiply the voltage applied to the specific power line by the current flowing through a current sensing coil which is coupled to a different power line.
In most cases, it is not known which of the voltage input lines and current sensing coils is connected to a given power line. This necessitates a great deal of trial and error in making proper connections between a watt/watthour meter and the power lines in a multiphase systems.
Additionally, the current running through a current sensor coil is often inverted in phase so that it is now 180.degree. C. out of phase with the voltage in the voltage input line which is connected to the same power line as the current sensing coil. If this phase-inverted current is multiplied by the voltage applied to the voltage input line, a negative power is incorrectly computed. To correct this problem, the phase of the current in each current sensing coil must be compared to the phase of voltage applied to the corresponding voltage input line. If a phase mismatch is detected between the current in one of the current sensing coils and its corresponding voltage, a phase invertor or other means for correcting current phase must be connected between the current sensing coil and the current input terminal so that the measured current is in phase with the corresponding voltage.
The process of current phase correction and the trial-and-error process of connecting the proper voltage input line or current sensing coil to each of the input terminals on the watt/watthour meter makes connection of a watt/watthour meter to a multiphase AC system a tedious and frustrating process. There is a longfelt need in the art for a watt/watthour meter of automatically correcting incorrect connections between the watt/watthour meter and the power lines in the AC system. Similarly, there is a longfelt need in the art for a watt/watthour meter which can automatically correct a phase mismatch between a current in a current sensing coil connected inductively to a power line and a voltage applied to a voltage input line which is connected to the same power line.
Although the preceding discussion of watt/watthour meters is limited to their use with a three phase, 4 wire AC system, everything that has been said is equally applicable to watt/watthour meters for use with three phase, three wire systems. The only difference in the connection between the watt/watthour meter and the system is that a third power line is disconnected from its voltage input and current input terminals and reconnected to the reference voltage input terminal. The voltage of each of the remaining power lines is then determined relative to the potential of the third power line. A neutral line is not present.